1. Field of the Invention
The present invention relates to a catalyst used for purifying the harmful substances such as hydrocarbons (HC), nitrogen oxides (NOx), carbon monoxide (CO), etc. present in the exhaust gas discharged from an internal combustion engine of automobile or the like.
2. Description of Related Art
Various catalyst techniques have heretofore been proposed for purification of the exhaust gas discharged from an internal combustion engine of automobile or the like. Recently, there have been proposed a number of catalysts comprising not only a noble metal (e.g. Pd) (which is an ordinary catalyst active component) but also an alkaline earth metal, an alkali metal or an oxide thereof. For example, in Japanese Patent Application Laid-Open No. 237384/1993, a catalyst is disclosed for exhaust gas purification obtained by coating, on a monolithic structure, a catalyst active component containing a noble metal (e.g. Pd or Rh), an alkaline earth metal oxide, a lanthanum oxide, a cerium oxide, a zirconium oxide and a heat-resistant inorganic oxide.
In recent years., it has been one of the most important technical tasks to effectively purify the unburnt combustible components (e.g. HC, CO and H.sub.2), in particular, HC discharged during the cold start of automobile engine. During the cold start, the engine is usually operated in a rich fuel-to-air ratio and HC is contained in the exhaust gas in a large amount; meanwhile, the catalyst used is still below the operating temperature and its purification ability is low; as a result, a large portion of HC tends to be discharged into the atmosphere without being purified. Hence, a technique is desired which can effectively purify the HC present in the exhaust gas discharged during the cold start of engine.
When the above-mentioned prior art is investigated from such a standpoint, the catalyst disclosed in Japanese Patent Application Laid-Open No. 237384/1993 uses an alkaline earth metal oxide in order to increase the Pd purifiability for NOx in a fuel-rich atmosphere; however, the literature pays little attention to the purification of HC and CO in a fuel-rich atmosphere.